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The invention relates to methods and apparatus for forming components from a powdered metal charge.
The forming of components from powdered metal is well known and widely used. The technique enables complex shapes to be formed to relatively high compacted densities of up to 75% of solid metal, at relatively low cost and with the dynamic mechanical properties of the component approaching those formed from solid metal blanks.
Conventionally, the powdered metal charge is compressed within a die by punches and the compacted component is then sintered in a sintering furnace to provide a durable component. In view of the forces encountered within the die and the flow of powder into the complex shapes defined by the die it is necessary to provide lubrication between the walls of the die and the powdered charge. Such lubrication reduces the frictional forces between the die walls and the powder and also reduces wear in the die.
Typically, the lubricant is incorporated into the powder but it has been found that this limits the upper value of the density that can be attained during compression.
It has also been proposed to lubricate the wall of a die prior to charging, but the lubricant tends to diffuse into the powder charge which is relatively porous. As a consequence, it is not available for lubrication of the die when required and may also produce inconsistencies in the material forming the charge.
A known method to overcome these disadvantages is to compress the powder containing the lubricant to readily attainable densities, typically 6.7-7.0 gm/cc for iron powder and its alloys, and then sintering the partially compressed component to burn off the lubricant contained within the powder. Following sintering, the component is further compressed to achieve a small increase in density for the final component. Such an arrangement does however require dual handling and dual sintering of the component which is generally undesirable.
It is therefore, an object of the present invention to provide a method and apparatus for forming a component from a powdered metal charge in which the above disadvantages are obviated or mitigated.
In general terms the present invention seeks to overcome the above disadvantages by partially compressing the powdered charge prior to bringing it into contact with the lubricated wall of the die. Accordingly, the powdered charge does not require the lubricant additive and the partially compressed component is not as absorptive of the lubricant as the uncompressed charge.
More specifically, according to one aspect of the present invention there is provided a method of forming an unsintered component from a powdered metal charge. The method comprises the steps of initially compressing the charge in a mold cavity to an intermediate density less than that of a desired final density, and then bringing a lubricated wall of a die into contact with the charge. Subsequently, the charge is compressed to the final density and removed from the die to provide an unsintered component.
Preferably, the lubricated wall of the die is moved into contact with the charge by displacement of the die relative to punches used for compression of the charge and as a further preference is maintained in the die between initial compression and subsequent compression.
According to a further aspect of the invention there is provided a method of forming a component from a powdered metal charge comprising the steps of establishing a mold cavity between an axial wall of a die and radial walls of a pair of opposed punches. The punches are positioned to locate the charge in a first zone of the axial wall of the die and compress the charge while maintaining it in contact with the first zone to an initial density less than a requisite final density. The punches are moved relative to the die to bring a second zone of the axial wall having a lubricant applied thereto in contact with the charge and the punches then compress the charge between the punches to the requisite final density. The compressed charge is then removed from the cavity.
The present invention also relates to a toolset for forming a component form a powdered metal charge. The toolset comprises a die and a pair of punches each slidable relative to the die and cooperating with the die to define a mold cavity. The die has an axial wall with first and second zones axially spaced a long the die. The die is selectively moveable relative to the punches to bring either the first or second zone into contact with a charge contained in said chamber.